I know there's the one in the millionaire center and it's 20 mil.

Are there any other cheaper options?

Sadly, that is the cheapest option, if I remember correctly. They go from 20 mil to 200 mil for 10 slots (20m increase each). Unless you count the , which you can choose once per year for free on your subeta anniversary.

Besides that, the only current way to buy slots is in the cash shop for 1500 csc. https://subeta.net/pets.php?act=buy_slots

Thank you. Wasn't sure if there were cheaper options so I appreciate the information.